1. Field of the Invention
This invention relates to a relay valve for a fluid braking system and more particularly to a pilot controlled air relay valve.
2. Description of Prior Art
Tractor-trailer combination vehicles are equipped with braking systems in which a vehicle operator generated a control signal by operation of the brake control valve in the operator's compartment. This control signal is transmitted to relay valves, which respond to the control signal to communicate compressed air from storage reservoirs to vehicle foundation brakes. The control signal must be communicated to the relay valves on the trailer through a pneumatic conduit which connects the tractor and the trailer.
More recently, so-called "turnpike trains" have become common, in which a single tractor pulls two or three trailers. Accordingly, the control signal generated by the vehicle operator must be transmitted through the first trailer to operate relay valves on the second and even a third trailer. However, when the control signal reaches the second and third trailers, it has become so slowed that unstable braking may result. Accordingly, it has become necessary to use pilot relay valves on the trailers to speed up the control signal. In other words, the control signal is used to operate a pilot relay valve, the outlet of which becomes the control signal transmitted to that trailer and each succeeding trailer. Obviously, it is desirable that such pilot relay valves used to generate control signal's have an extremely small pressure differential between the control port and the outlet or delivery port, to provide nearly equal pressure at all brakes. It is also desirable that such relay valves react as quickly as possible to the control signal transmitted to the control port to minimize time delay. Accordingly, it is desirable that the crack-open pressure, at which such a relay valve opens, be as small as possible to provide adequate responsive control during braking.
A pilot relay valve is disclosed in U.S. Pat. No. 5,261,458 which is specifically designed to accelerate the brake control signal on air-braked vehicles while maintaining a zero control/delivery pressure differential. This pilot relay valve includes a relay piston, which is operable in response to the control signal to open the valve, having an orifice there through which permits the control pressure volume to communicate with the valve outlet. The orifice permits pressures to equalize so that pressure communicated through the outlet port will equalize or balance with the pressure at the control port, to provide and maintain a zero pressure differential between the control port and the outlet port.
Non-pilot relay valves do an excellent job of accelerating control signals but generally cause pressure differentials, typically 2 to 4 psi, which directly relate to air brake system imbalance. This system imbalance or pressure differential is additive on double or triple trailer combinations and is a direct cause of vehicle instability. The inventors have observed that a pilot relay valve, as described above, while maintaining zero control/delivery pressure differential is somewhat slower than a non-pilot relay valve, primarily during release, due partially to the balance orifice in the relay piston. On some vehicle combinations, such as double and triple trailer units, brake response timing and dynamic balance are generally less than desirable as far as system transmission timing is concerned.